From mboxrd@z Thu Jan  1 00:00:00 1970
From: Julien Grall <julien.grall@citrix.com>
Subject: Re: Design doc of adding ACPI support for arm64 on Xen
 - version 5
Date: Wed, 2 Sep 2015 13:52:32 +0100
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To: Shannon Zhao <shannon.zhao@linaro.org>, Shannon Zhao <zhaoshenglong@huawei.com>, xen-devel <xen-devel@lists.xen.org>
Cc: Hangaohuai <hangaohuai@huawei.com>, Ian Campbell <ian.campbell@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, andrew@fubar.geek.nz, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>, Stefano Stabellini <stefano.stabellini@citrix.com>, David Vrabel <david.vrabel@citrix.com>, Jan Beulich <jbeulich@suse.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Parth Dixit <parth.dixit@linaro.org>, Christoffer Dall <christoffer.dall@linaro.org>, Roger Pau Monne <roger.pau@citrix.com>
List-Id: xen-devel@lists.xenproject.org

On 02/09/15 13:02, Shannon Zhao wrote:
>> Hold on, this is about Linux able to use the memory for his own usage.
>> ACPI table are not part of this memory because they are marked reserved
>> by the firmware.
>>
>> If we follow your logic, all ACPI tables always should be above the
>> kernel. I don't believe this is the case and it would be buggy on Xen
>> because of the DOM0 direct RAM mapping (i.e the first RAM bank can be
>> very high and the kernel too).
>>
> 
> It looks weird. But from the booting.txt, it says the memory below base
> address is unusable and from early_init_dt_add_memory_arch in Linux, it
> really ignores the memblock below the PAGE_OFFSET.

It's unusable in the sense that Linux can't use them to store its own
data. Having the ACPI table outside of the System RAM is valid because
they are marked as reserved and ioremap will be used to map them.

> 
>> I think the problem is how you reserved this region in the EFI memory
>> table. From what I saw, you marked this new memory with EFI_MEMORY_WB
>> (which means that the region can be usable by Linux).
>>
> Yes, I mark it with EFI_MEMORY_WB. Is this right?

I would say no, but it's only because I looked at the kernel code quickly.

You have to looks how ACPI region/UEFI tables are described in the host
EFI memory map and mimicking for the DOM0 EFI memory map.

Regards,

-- 
Julien Grall